Nonwoven fabrics were developed in an attempt to produce an inexpensive fabric by eliminating many of the various steps required to produce woven or knitted fabrics. Initially, nonwoven fabrics were produced from card or air-laid webs of fibers which were bonded with a chemical binder. Such fabrics have relatively limited usage because their strength characteristics were poor compared to woven or knitted fabrics and their absorbency and softness characteristics left something to be desired because of the use of chemical binders. Major advances were made in eliminating or considerably reducing the amount of binder used in a nonwoven fabric by rearranging or entangling the fibers in a fibrous web to produce what are termed xe2x80x9cyarn likexe2x80x9d fiber segments and entangled fiber areas. Methods and apparatus for producing fabrics of this nature are more fully disclosed in U.S. Pat. Nos. 2,862,251, 3,033,721, and 3,486,168. While these techniques improve the strength characteristics of nonwoven fabrics, they still did not have the strength characteristics of the woven or knitted fabrics. These entangled or rearranged fiber fabrics did require less binder and, hence, had good absorbent characteristics and excellent softness. As a result of this, nonwoven fabrics found primary uses in many products such as sanitary napkins, disposable diapers, replacement gauze, medical bandages, and the like. While such products were accepted for uses where absorbency and softness was desired, the various different fiber areas would absorb differently. For example, yarn-like structures would absorb different than non-yarn-like structures. Furthermore, many of these fabrics included apertures or holes and while suitable for facing materials, were not suitable for some absorbent products unless used in multi-layer configurations. While nonwoven fabrics have gained wide acceptance, it is still desired to improve the absorbent characteristics of such fabrics and make them more efficient in use.
It is an object of the present invention to produce a nonwoven fabric having improved absorbent characteristics. It is a further object of the present invention to produce a nonwoven fabric having relatively uniform absorbent characteristics. It is still a further object of the present invention to produce a nonwoven fabric that has improved absorbent characteristics without any deleterious effects on the other desired properties of nonwoven fabrics.
Nonwoven fabrics of the present invention have substantially uniform absorbent characteristics in all directions within the plane of the fabric. The nonwoven fabric has a repeating pattern of three interconnected fiber arrays. The first fiber array of the fabric comprises a plurality of parallel fiber segments. The second fiber array comprises a plurality of twisted and turned fiber segments that form a band disposed substantially perpendicular to the parallel fiber segments of the first fiber array. The second fiber array is disposed adjacent the first fiber array. The nonwoven fabric of the present invention includes a third fiber array which interconnects the first and second fiber arrays. The third fiber array comprises a plurality of highly entangled fiber segments.
Nonwoven fabrics of the present invention have uniform absorbent characteristics such that the pattern of absorption of fluid by the fabric has a mean roundness factor of 0.6 or greater. Also, the pattern of absorption has a generally smooth perimeter such that it has a mean form factor of 0.7 or greater.
It is believed these combined absorbent properties of the fabrics of the present invention may result from the unique distribution and configuration of fiber in the fabric. Nonwoven fabrics of the present invention have a generally sinusoidal fiber distribution curve over their cross-sectional area. This generally sinusoidal fiber distribution curve of the fabrics of the present invention must meet certain criteria. We have found that one way of defining and measuring these criteria is by mathematically defining the fiber distribution curve. The curve may be defined by the average percentage of area covered by fibers, the cycles or periodicity of the curve and the average amplitude of the curve. We have found that the fabrics of the present invention have a fiber distribution index of at least 600 and preferably at least 800. This fiber distribution index is determined by multiplying the average percentage of area of fiber coverage in a specific measured cross-sectional area of the fabric by one-half the number of clearly identifiable points of minimum fiber coverage over said specific cross-sectional area and dividing this figure by the average amplitude of the fiber distribution curve.